For Mario Vargas Llosa, a novelist has the obligation to question real life. ''I don't think there is a great fiction that is not an essential contradiction of the world as it is,'' he said during a recent visit to New York. The writer creates an alternative world in response. ''This is,'' he said, ''the great contribution of the novel to human progress. You know, the Inquisition forbade the novel for 300 years in Latin America. I think they understood very well the seditious consequence that fiction can have on the human spirit.''

Mr. Vargas Llosa was in a conference room at the Americas Society, the sponsor of his lecture the following evening on ''Literature and Life,'' his two favorite subjects, or rather, his favorite subject. For him, literature and life are inextricable: each nourishes the other.

Turning 66 today and at the peak of a thriving career on several continents -- he has homes in London, Madrid and Lima, in his native Peru -- he exudes confidence and well-being. He has been greatly honored for his novels and essays; among major awards only the Nobel Prize for literature has eluded him. Through his work, he has revealed himself as a concerned and committed writer as thinker. His latest book, ''The Feast of the Goat'' (Farrar, Straus & Giroux), is one of his most political novels, a fictionalized treatment of Gen. Rafael Trujillo and his 31-year reign of terror in the Dominican Republic, which ended with his assassination.

Speaking in English with a pronounced Spanish accent, Mr. Vargas Llosa wondered why the countries of Latin America have had so many dictators -- and mediocre politicians -- and at the same time, so many great writers have emerged. He did not name the writers, but he could have been thinking about Gabriel García Márquez, Jorge Luis Borges and, of course, himself, among others who comprised what was called the Boom, the explosion of creativity in Latin America in the 1960's. Analyzing the proliferation of dictatorships, he said that many citizens are willing, and sometimes eager, to surrender their free will, and that the artist must challenge that authority.

In his own life, Mr. Vargas Llosa has been a politician, albeit briefly. In 1990, he ran for the presidency of Peru. Although he began as the favorite, he lost to Alberto Fujimori, who, after he was elected, became a corrupt and authoritarian leader. Finally in 2000, he was driven out of power and went into exile in Japan.

Asked why he wanted to be president, Mr. Vargas Llosa said, ''Because I was an idiot,'' and laughed heartily. ''I don't think there is another explanation. I thought I could help. I was very naïve, but I learned a lot. It was very instructive.''

If he had become the president, he said, he would have stopped writing novels for five years. In other words, he would not have written -- or at least, he would have postponed -- writing the books that became ''Death in the Andes,'' ''The Notebooks of Don Rigoberto'' and ''The Feast of the Goat.'' That would have been the world's loss, but had he defeated Mr. Fujimori, it undoubtedly would have been Peru's gain.

''I would have tried to do what I promised,'' he said, ''and there wouldn't have been the tremendous corruption and brutality of these last 10 years in Peru. But who knows? One of the reasons I went into politics was because I thought that democracy was so fragile that it could collapse, and that would be a tragedy.''

''The Feast of the Goat'' is, of course, about one such tragedy, the Goat being a nickname for Trujillo. Three narrative strands run concurrently in the novel. At the center is Trujillo in power in 1961, as the moment of his assassination approaches. Outside the dictator's mansion are the men who are plotting against him, some of them former aides who have finally risen in protest. In the present is the daughter of a discredited Trujillo official. She has returned to Santo Domingo after a long self-exile in the United States. The author calls these stories trajectories, signifying the propulsive journeys undertaken by the characters.

Asked how closely the novel hewed to reality, he said: ''It's a novel, not a history book, so I took many, many liberties. The only limitation I imposed on myself was that I was not going to invent anything that couldn't have happened within the framework of life in the Dominican Republic. I have respected the basic facts, but I have changed and deformed many things in order to make the story more persuasive -- and I have not exaggerated.'' In other words, truth is even stranger than fiction.

He traced the genesis of the novel to 1975 during a prolonged visit to the Dominican Republic, when he gathered stories about Trujillo. About five years ago, he began writing.

His Trujillo, he said, is a portrait from life: ''He had more or less all the common traits of a Latin American dictator, but pushed to the extreme. In cruelty, I think he went far far away from the rest -- and in corruption, too.'' He was, he said, a showman, and during his rule, life in the Dominican Republic ''was a kind of operatic farce, orchestrated by this man who had practically total control and could convert people into actors in a very sinister show.''

Money was not an important factor to him, but power and sexual potency were essential: ''He went to bed with his ministers' wives, not only because he liked these ladies but because it was a way to test his ministers. He wanted to know if they were ready to accept this extreme humiliation. Mainly the ministers were prepared to play this grotesque role -- and they remained loyal to Trujillo even after his death.''

Mr. Vargas Llosa also discovered that Trujillo claimed young women, virgins, by droit du seigneur, as if he were a feudal lord: ''Many fathers, modest people, peasants, brought their daughters as a gift to el Jefe -- and they were very proud, not only because they wanted favors but because it was also an honor. He was a god, and so everything that touched the god was of value.''

At the same time, the author held to his goal, to depict Trujillo not as a monster but as what he calls ''a human being transformed into this monstrous being not only because of his excesses, but because of this abject subordination of the people.''

After the assassination, the subsequent coup failed. The quiet hero of the novel is Joaquín Balaguer, a poet and colorless functionary whom Trujillo had installed as his puppet president because he wrongly thought Balaguer had no ambitions. By retaining his composure, Balaguer managed to unify the country.

When Mr. Vargas Llosa was writing his novel he had three long conversations with Balaguer. ''He was so clever to evade difficult questions. He was -- how do you say it? -- an anguila, an eel. I said to him: 'Dr. Balaguer, you are a cultivated man. How could you serve a gangster with such loyalty and competence for 31 years?' He answered that he wanted to be a politician and could only do so in the Dominican Republic by working for Trujillo, but vowed not to participate in sexual orgies with Trujillo and not to steal one dollar. Proudly he said he kept his vow.

After finishing ''The Feast of the Goat,'' Mr. Vargas Llosa began work on a new novel, about Flora Tristan, Paul Gauguin's grandmother, whom he calls ''a social agitator with a tragic history, probably the first real feminist in Europe.''

''Her personality was very similar to her grandson,'' he said. ''They were both stubborn and idealistic.'' Each looked for a paradise on earth, Tristan through social justice, Gauguin, through beauty, which he searched for in primitive cultures.

Mr. Vargas Llosa is known for his essays as well as his novels, and often speaks out on literary as well as political issues. In one essay, he defined Borgesian as an entry into a fantastical universe, and also offered definitions of Kafkaesqe and Orwellian. Asked to define Vargas Llosian, he said: ''I could answer with a Borgesian quotation: 'When you look at yourself in a mirror, you don't know if you are handsome or very ugly.' I cannot measure my books with even the minimal objectivity.''

But he talked about what he was attempting to do: ''As a novelist, I want to create a world that can be persuasive by itself, by its language, mythology, the strength of its characters.'' When he wants to defend cultural or political or social ideas, he writes essays. ''For me, fiction is something much more mysterious, and comes from very deep images,'' he said. He added that when he is writing a novel, his whole personality is involved: ''ideas, but also instincts, intuitions -- and passion.''

When he is not writing or giving speeches, he spends his time reading. ''I was a reader before I was a writer,'' he said, and he remains loyal to his favorites, Flaubert, Faulkner and Melville. ''I am convinced that what I am now owes a lot to the great writers I have read. Without those books, I would be a poorer person.''